The Ultimate Guide 1947-1954 GMC and Chevrolet Trucks

12 Volt Conversions for Classic Chevy and GMC Trucks

A conversion from 6 to 12 volts is one of the simplest and most rewarding upgrades you can give your classic truck. A modern 12v electrical system will give you easier starting, brighter headlights, and compatibility with other components like power accessories or stereo equipment. The conversion can be done in a few hours and the required parts are relatively inexpensive. These instructions apply specifically to the 1947 – 1954 Advance Design Chevy and GMC trucks, but the procedure outlined here can also be applied to other makes and models.

Conversion Options

The conversion can be done using either a 12v generator, or a 12v alternator:

A 12v generator will look original, and can be mounted on a Chevy 6 with some very minor alterations to the brackets. You can find a replacement 12v generator, or have field coils replaced in the existing 6v generator case. Mid 50s Chevy cars came from the factory with 12 volt generators mounted on a 235, and make great replacements. The disadvantages of using a generator are minor and are not usually an issue. Any generator system will show a slight discharge at idle, and most alternators are capable of delivering more amps than a generator system.

A seemingly more widely used approach is to replace the generator with a modern alternator with an internal regulator. An alternator will generally provide more power with less maintenance than a generator, but some originality must be sacrificed.

Before we begin

Before we dig in to the conversion, lets establish a few facts:

Your original 6V wiring harness will work! A 6V electrical system carries twice the amps of a 12V system, so your existing wiring is actually heavier than necessary.

You can use your original 6V starter. It works great!

Switch your GMC to Negative Ground

While you’re at it, this is a great time to convert your GMC with POS ground to NEG ground. (Chevrolet trucks are already negative ground) This switch can be done in a few simple steps:

Disconnect the battery.

Reverse the wires on the Ammeter. “In” becomes “Out” and vice-versa.

Reverse the wires on the coil. Ignition is now (+)

Reconnect the battery cables in reverse. POS will now go to the starter. NEG will be attached to the ground strap.

Make a Parts List

The following items will be required, regardless of which conversion method you use. Patrick’s Antique Carshas all the parts you’ll need for the conversion, but you can also find them from other vendors or your local auto parts store.

Some accessories, such as power solenoids and radios must be replaced or converted to 12 volts. A simple resistor will not work. If your vehicle has an ammeter, it will work properly on 6 or 12 volts without any modifications.

Install the above listed Gas Gauge Voltage Reducer on the rear of the gauge.

Install the above listed Accessory Voltage Reducer in the wire between the heater and heater switch.

Make sure all other accessories are 12-volt ready.

Reconnect the Battery and you’re done!

The Conversion with an Alternator

An alternator conversion is slightly more complicated than generator conversion, but it’s still pretty straightforward. (Again, all parts are available from Patrick’s Antique Cars)

Disconnect the battery.

Remove the 6V generator.

Mount the Alternator. Brackets are available from various manufacturers.

Use a “DA Plug” which allows you to eliminate the old voltage regulator. Attach the short wire to the alternator post. Attach the long DA wire to the ignition switch so it is powered only when the switch is on.

Remove the voltage regulator, and connect the BATT wire to the wire that previously ran to the generator. Attach this to the alternator post (along with the short DA wire).

Replace the 6V Coil with a 12V Coil. Use a coil with an internal resistor or install an ignition ballast resistor in the wire between the coil and the ignition switch.

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Comments (4)

darrell

2 years ago

have a 1952 chevy pickup that has been changed to 12 volts. ordered a 12 volt gas gauge, wired it the same as the 6 volt does not work. ordered 2nd one, still will not work. there is current to the gas gauge and the wire going to gas tank checks out.Is there something else that need to be done?

Ron

1 year ago

Changed a 6 volt gas gauge to 12volt. The needle goes all the way off the gauge and points 180 degree away from the gauge.

al

11 months ago

what if you use a one wire alternator.

Ryan

10 months ago

With a 12 volt gauge, most require ground, power and signal. So, positive to gauge which you said you have. Signal wire from tank which you also mention was good. Ground wire to gauge which I didn’t see mentioned. Also you level will need to have those three as well. If you’re hooked up right and you ground out the signal wire, your gauge should spike to full. If this works then there is something wrong with your sending unit.